The timing and technique of the injection of these tracers for sentinel node localization has been extensively researched. The procedure typically begins with the injection of the Tc99; a radiotracer bound to a colloid substance that travels through the lymphatic system. Sulfur colloid is the molecule commonly used in the United States;albumin is often the preferred com­pound overseas. In the United States, Tc-99m sulfur colloid is available as unfiltered or fil­tered, having been passed through a 22-mm filter. The uptake and travel time depend on the size of the labeled carrier and the amount of carrier fluid used. Larger particles may never make it to the nodes, whereas small particles may go too quickly, possibly resulting in multi­ple positive nodes. Filtration eliminates much of the heterogeneity found in the sulfur colloid molecules, theoretically producing a more con­centrated and easily localized radioactive signal when explored with a gamma probe. Filtration through 100- or 220-nm filters has been stud­ied, with goals of particle sizes ranging from 50 to 200 nm. A number of studies have been performed, examining the clinical benefit of using filtered versus unfiltered Tc-99m. The results have failed to demonstrate a clear advantage of one over the other. Selection may depend on the relative advantages and dis­advantages, including when the injection is performed (day of versus night before).

Often the injections are performed the morn­ing of the surger y, with lymphoscintigraphy performed 2 hours after injection. This can complicate surgical scheduling because cases involving SLN biopsies cannot begin until late morning. Several studies have demonstrated no difference in node identification rates using Tc-99m between 2 and 24 hours after injection, a fact that often simplifies the logistics of sche­duling surgery by allowing the injection to take place the night before.